1. Field of the Invention
The present invention relates to a stencil printer equipped with a master making section, and more particularly to structure for applying back tension in wrapping a perforated stencil paper (hereinafter referred to as a perforated master sheet) around a master pinch drum.
2. Description of the Related-Art
A conventional stencil printer equipped with a master making section is shown, for example, in Japanese Patent Publication No. 2538817. Master conveying rollers and master discharging rollers are provided behind the master making section to warp a perforated master sheet around a master pinch drum after the perforated master sheet has been made. When making the perforated master sheet, the master discharging rollers are stopped and the master conveying rollers are rotated to slacken the perforated master sheet between the discharging and conveying rollers and store the slack part therebetween. At the time the master making process has ended, the master pinch drum with the leading end of the master sheet clamped to the clamp portion of the drum is rotated to wrap the master sheet around the drum, while tension is being applied to the master sheet by the discharging rollers.
In the process of wrapping the master sheet around the drum, it is important from the standpoint of obtaining an accurate master-wrapped state to apply stable back tension to the master sheet when the master sheet, the leading end of which is clamped to the clamp portion of the drum, is wrapped around the drum by rotation of the drum. However, in the aforementioned stencil printer, there is a danger that stable back tension cannot be applied to the master sheet, because back tension is applied only by rotation of the discharging rollers resulting from movement of the master sheet.
That is, when the perforated master sheet is wrapped around the master pinch drum, the master sheet has been slackened and stored on an upstream side from the discharging rollers. In this state, in the case where back tension is applied to the master sheet only by the rolling resistance which occurs due to rotation of discharging rollers caused by movement of the master sheet, a certain degree of back tension can be applied at the moment the master sheet starts moving from the stopped state thereof to rotate the master discharging rollers. However, after the discharge rollers have started rotating, it is mechanically difficult to apply stable back tension to the master sheet because of load fluctuation in the rotation of the discharging rollers.
The present invention has been made in view of the aforementioned problems found in the aforementioned stencil printer. Accordingly, the primary object of the present invention is to provide a stencil printer which is capable of performing an accurate master wrapping operation by applying stable back tension when a master sheet with the leading end clamped to a master pinch drum is wrapped around the drum.
To achieve this end and in accordance with one important aspect of the present invention, there is provided a stencil printer wherein an elastic member is provided on a master conveying path from a roll of unperforated master sheet to a master pinch drum, the elastic member being pressed against a perforated master sheet to apply back tension to the perforated master sheet when the perforated master sheet is moved by rotation of the master pinch drum after the leading end of the perforated master sheet has been clamped to the clamp portion of the master pinch drum.
In accordance with another important aspect of the present invention, there is provided a stencil printer wherein
along a master conveying path from a roll of unperforated master sheet to a master pinch drum, a master making section, master conveying rollers, and master discharging rollers are disposed in order so that a perforated master sheet is slackened between the master conveying rollers and the master discharging rollers; and
an elastic member is provided between the master discharging rollers and the master pinch drum, the elastic member being pressed against the perforated master sheet to apply back tension to the perforated master sheet when the perforated master sheet is moved by rotation of the master pinch drum after the leading end of the perforated master sheet has been clamped to the clamp portion of the master pinch drum.
In accordance with still another important aspect of the present invention, there is provided a stencil printer wherein
along a master conveying path from a roll of unperforated master sheet to a master pinch drum, a master making section, master conveying rollers, and master discharging rollers are disposed in order so that a perforated master sheet is slackened between the master conveying rollers and the master discharging rollers; and
an elastic member and a smoothing member are provided between the master discharging rollers and the master pinch drum, the smoothing member being fixed with the stencil printer, the elastic member being pressed against the smoothing member through the perforated master sheet to apply back tension to the perforated master sheet when the perforated master sheet is moved by rotation of the master pinch drum after the leading end of the perforated master sheet has been clamped to the clamp portion of the master pinch drum.
In a preferred form of the present invention, the master discharging rollers apply no load to the perforated master sheet being moved, when the elastic member is in pressing contact with the perforated master sheet.
In another preferred form of the present invention, a mechanism for adjusting a force that the elastic member is pressed against the smoothing member through the perforated master sheet is provided.
According to the present invention, an elastic member is provided on a master conveying path from a roll of master sheet to a master pinch drum and is pressed against the master sheet being moved by rotation of the master pinch drum so that back tension is applied to the master sheet. With this arrangement, a constant and stable back tension can be applied to the master sheet being moved, by the sliding resistance which occurs when the elastic member is pressed against the master sheet. As a result, master-wrapping precision is enhanced, whereby printing quality can be enhanced.
The aforementioned tension blade is disposed between the discharging rollers, which slackens and stores the perforated master sheet, and the master pinch drum. With this arrangement, stable back tension can be applied even at the final stage of wrapping the master sheet around the drum and therefore an accurate master-wrapping operation can be performed. Furthermore, since the master discharging rollers apply no load to the master sheet when the tension blade is in pressing contact with the master sheet, load fluctuation resulting from rotation of the discharging rollers caused by movement of the master sheet can be suppressed and therefore stable back tension can be applied to the master sheet.